1. Field of the Invention
The present invention relates to light strings, and, more particularly, to light strings employing LEDs.
2. Description of the Related Art
Light emitting diodes (LEDs) are increasingly employed as a basic lighting source in a variety of forms, such as outdoor signage and signaling, replacement light bulbs, or decorative lighting, for the following reasons. First, as a device, LEDs have a longer lifespan than all other standard light sources, particularly common, fluorescent and incandescent sources, with typical LED lifespan, being at least 200,000 hours, as measured by 30% loss of light output degradation over time. Second, LEDs have several favorable physical properties, including ruggedness, cool operation, ability to operate under a wide temperature variation, and safe low-voltage power requirements. Third, newer, more sophisticated doping technologies, increase LED efficiency measured as light output versus power consumed, with efficiencies on the order of ten times that of incandescent lighting. Fourth, LEDs are becoming increasingly cost effective with the increase in applications and resulting volume demand. Fifth, blue LEDs allow full-color or adjustable-color lighting by employing a red/green/blue (RGB) sub-die combination. Sixth, wideband "white" LEDs and related phosphoring technologies allow white LEDs to have a white-light output of good color rendering index without employing a RGB sub-die combination.
LED-based light strings, such as decorative Christmas tree lights, is one such application for LEDs. For example, U.S. Pat. No. 5,495,147 entitled LED LIGHT STRING SYSTEM to Lanzisera (hereinafter "Lanzisara") and U.S. Pat. No. 4,984,999 entitled STRING OF LIGHTS SPECIFICATION to Leake (hereinafter "Leake") describe different forms of LED-based light strings. In both Lanzisera and Leake, exemplary light strings are described employing purely parallel wiring of discrete LEDs with a step-down transformer and rectifier power supply. These light strings of the prior art convert from 110 VAC to DC voltage required to drive a single LED in the string and assume that all LEDs in the light string have the same drive voltage. Further, Leake employs a special LED package with two short, sharpened leads bridging across and penetrating the two soft insulated wires of the light string. Lanzisera employs a complex power supply incorporating not only a step-down transformer and rectifier, but also a zener diode and voltage regulator. In addition, Lanzisera describes connecting multiple strings of LEDs in parallel end-to-end using a polarized connector and regulator to provide constant voltage and current.